In the electrophotographic process, an image comprising an electrostatic field pattern, usually of non-uniform strength (also referred to as an electrostatic latent image), is formed on an insulative surface of an electrostatographic element by any of various methods. For example, the electrostatic latent image may be formed electrophotographically (i.e., by imagewise photo-induced dissipation of the strength of portions of an electrostatic field of uniform strength previously formed on a surface of an electrophotographic element comprising a photoconductive layer and an electrically conductive substrate), or it may be formed by dielectric recording (i.e., by direct electrical formation of an electrostatic field pattern on a surface of a dielectric material). Typically, the electrostatic latent image is then developed into a visible image by contacting the latent image with an electrostatographic developer. If desired, the latent image can be transferred to another surface before development.
One well known type of electrostatographic developer comprises a dry mixture of toner particles and carrier particles. Developers of this type are commonly employed in well-known electrostatographic development processes such as cascade development and magnetic brush development. The particles in such developers are formulated such that the toner particles and carrier particles occupy different positions in the triboelectrical continuum, so that when they contact each other during mixing to form the developer, they become triboelectrically charged. The toner particles thus acquire a charge of one polarity and the carrier particles acquire a charge of the opposite polarity. These opposite charges attract each other such that the toner particles cling to the surfaces of the carrier particles. When the developer is brought into developing, e.g., contact relation, with the latent electrostatic image, the electrostatic forces of the latent image (sometimes in combination with an additional applied field) attract the toner particles, and the toner particles are pulled away from the carrier particles and become electrostatically attached imagewise to the latent image-bearing surface. The resultant toner image can then be fixed in place on the surface by application of heat or other known methods or can be transferred to another surface to which it then can be similarly fixed.
A more recent development of the electrostatographic process is its application for the production of colored images. These systems are based on trichromatic color synthesis such as produced by subtractive color formation wherein at least three separate color separation images are formed and the combined images brought into register with each other to form a colored reproduction of a full colored original.
In accordance with such a process, a photoconductor having a uniformly charged photoconductive surface (photorecptor) capable of forming an electrostatic latent image is exposed through a green filter to an imagewise projection of a color image to form an electrostatic latent image on the photoreceptor. This electrostatic latent image is then developed with the complementary magenta color toner to form a magenta colored image corresponding to said electrostatic latent image, and transferred in register to an image receiving member. The photoreceptor is then electrostatically charged uniformly in the dark and exposed through a red filter to an imagewise projection of a color image in register with said magenta developed image to form a second electrostatic latent image, which second image is developed with the complementary cyan color toner and likewise transferred in register. The photoconductor is again electrostatically uniformly charged in the dark and then exposed through a blue filter to an imagewise projection of a color image in register with said magenta and cyan developed images to form a third electrostatic latent image which is then developed with the complementary yellow toner and again transferred in register. The sequence of exposures through colored filters in this multiple development process may be performed in any suitable sequence other than the green, red and blue mentioned.
This combination of three color toner images is generally made on a copy sheet transfer member such as paper or clear plastic to which the toner images are permanently affixed. One of the most common techniques for fixing these toner images to the copy sheet comprises employing a fusible resin toner which includes a colorant, and heat fixing the toner images to the copy sheet. Images may also be fixed by other techniques such as for example, subjecting them to a solvent vapor.
In most regions of the transfer member, the transferred layers are coated one on top of the other, the first layer being the magenta layer, the second being the cyan layer and the third being the yellow layer. Each substractive color transmits two thirds of the spectrum and absorbs one third. The combination of cyan, magenta and yellow layers appears black, while the combination of magenta and yellow layers appears red, the combination of magenta and cyan layers appears blue, and the combination of yellow and cyan layers appears green. Images of enhanced contrast may also be prepared using a fourth black-colored toner in combination with the cyan, magenta and yellow toners described above.
In the color process, a colorant and resin combination used to make the toner must meet a number of stringent requirements. First, the formulated colorant must be of the correct color and hue within its spectral band width with minimal response in other bands so that it works together with complementary colorants to produce faithful color reproductions. Second, a colorant and resin combination must be selected such that the toner possesses the appropriate triboelectric properties which will enable it to function and continue to function in an electrophotographic imaging mode. It is a function of the toner-carrier combination or developer package in a given development system, for example cascade, to assume a triboelectric relationship such that the toner will be carried with the carrier during the development cycle by electrostatic attraction and then be selectively deposited charge-wise on the electrostatic latent image which has a greater affinity for the toner electrostatically than does the carrier particle. In addition to the very significant triboelectric properties that a developer must possess and maintain during the development cycle, the toner must not only possess the appropriate color, but must also continue to function under machine conditions which expose the developer to impaction, humidity and oxygen among other undesirable factors. Third, the colorant used in formulating colored toner must be sufficiently color stable such that toner material of the requisite particle size, i.e., preferably from about 2 to about 20 microns, can be prepared without any significant diminution of color properties of the toner such as spectral response, color mixing characteristics and transparency. Finally, toners containing such colorants must also exhibit good transfer characteristics and the colorants preferably should have good heat and light stability.
Quite clearly, requirements must be met for the provision of colored toner materials capable of reproducing faithful electrostatographic reproductions of colored originals which are more stringent than the fact that a particular pigment used to produce the toner may be of a magenta, cyan or yellow color.